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Understanding Cancer Series

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  • Posted: 12/29/2009

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NCI Highlights

Slide 7

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Genetic Variations Called SNPs

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Cancer researchers think that, in addition to inherited or acquired mutations, tiny variations in a person's genome called single nucleotide polymorphisms, or SNPs ("snips") for short, may play a role in cancer. A SNP is defined as a single base change in a DNA sequence that occurs in a significant proportion (more than 1 percent) of a large population. The single base is replaced by any of the other three bases. Here is an example: In the DNA sequence TAGC, a SNP occurs when the G base changes to a C, and the sequence becomes TACC.

SNPs are scattered throughout a person’s genome and are found in both coding AND noncoding regions. They can cause silent, harmless, harmful, or latent effects. They occur with a very high frequency, with estimates ranging from about 1 in 1000 bases to 1 in 100 to 300 bases. This means that there could be millions of SNPs in each human genome.

Graphic shows how two bases can switch places at a particular spot such as a G to C, and if it happens in more than 1% of a large population at that spot, the change is called a single nucleotide polymorphism.